The present invention relates to an exhaust gas recirculation system for a diesel internal combustion engine, and more particularly relates to such an exhaust gas recirculation system which reduce or eliminate smoke in the exhaust of the diesel internal combustion engine during rapid acceleration thereof.
Diesel internal combustion engines are nowadays widely used for propulsion of automotive vehicles, and the small scale type of diesel internal combustion engine suitable for small scale types of automotive vehicle such as passenger cars and light trucks is becoming more and more popular. For minimizing the exhaust emissions of diesel internal combustion engines, including particularly minimizing the production of nitrogen oxides or NOx in said exhaust emissions, and also including minimizing the amount of smoke or soot in said exhaust emissions, the method of exhaust gas recirculation is well known and effective. In this method, a part of the exhaust gases of the diesel internal combustion engine is abstracted from the exhaust system thereof and via an exhaust gas recirculation passage is fed into the air intake passage to mix with the air which is being sucked in therethrough to enter the combustion chambers of the diesel internal combustion engine and to be therein combusted together with liquid fuel injected into said combustion chambers. Thus this recirculated exhaust gas replaces a part of the air that would otherwise be sucked into these combustion chambers.
This admixture of exhaust gases into the air which is being sucked into the diesel internal combustion engine is effective for controlling the emission of harmful pollutants in the exhaust gases, including particularly for controlling the emission of smoke and soot in these exhaust gases, and also preserves good drivability of the diesel internal combustion engine, but it is necessary to regulate the amount of recirculated exhaust gases according to various criteria relating to the operational state of the diesel internal combustion engine. Generally, in the case of a diesel internal combustion engine, the so called excess air ratio, defined as (A-R)/R, where the symbol A represents the amount of air being sucked into the diesel internal combustion engine, and the symbol R represents the amount of air that is actually required to combust the liquid fuel which is being injected into the combustion chambers thereof, decreases along with increase in engine load.
Now, in the case of a diesel internal combustion engine which is being provided with exhaust gas recirculation, it is not proper to admit more recirculated exhaust gases to the air intake system of the engine than this amount of excess air, because otherwise the amount of air in the combustion chambers of the diesel internal combustion engine will become insufficient to combust the amount of fuel that is being injected into said combustion chambers, and accordingly incomplete combustion of this fuel will result, which will cause the emission of serious quantities of HC and CO and other undersirable hydrocarbons in the exhaust gases of the diesel internal combustion engine, as well as the emission of considerable quantities of soot and smoke therein. Therefore the process of exhaust gas recirculation must be controlled so that the exhaust gas recirculation ratio should decrease in response to an increase in engine load.
One way in which this control of exhaust gas recirculation has been performed in the past is to sense the position of a fuel metering element of the diesel fuel injection pump of the diesel internal combustion engine, i.e. the amount of rotation of a spill ring therein or the amount of rotation of a pump operating lever or the like, and according to this position, and also according to other engine operating parametrs such as temperature of the cooling water, temperature of the intake air, pressure in the intake air passage, engine revolution speed, etc., to generate a control fluid pressure for operating a fluid pressure operated exhaust gas recirculation control valve which includes a diaphragm actuator and controls the effective flow resistance of the exhaust gas recirculation passage which recirculate the exhaust gases as explained above.
However, a difficulty has arisen with regard to the operation of such a prior art exhaust gas recirculation control system, as follows. The diesel fuel injection pump has a relatively quick response to change of position of the fuel metering or controlling element thereof, and accordingly when the accelerator pedal of the vehicle (which controls the position of said fuel metering or controlling element) is rapidly depressed in order rapidly to accelerate the vehicle which is powered by the diesel internal combustion engine, then the engine load, i.e. the amount of diesel fuel injected per one fuel injection pulse by the fuel injection pump, increases very quickly. However, the fluid pressure operated exhaust gas recirculation control system has a relatively slow response to change of position of the fuel metering or controlling element of the fuel injection pump, and accordingly when the accelerator pedal of the vehicle which controls the position of said fuel metering or controlling element is rapidly depressed in order rapidly to accelerate the vehicle which is powered by the diesel internal combustion engine, then the amount of exhaust gas recirculation provided by said exhaust gas recirculation control system decreases rather slowly. Thus, a tendency exists for the amount of exhaust gas recirculation not to be decreased quickly enough corresponding to the increase in fuel injection amount, when the accelerator pedal of the vehicle is rapidly depressed in order rapidly to accelerate the vehicle, and this means that during said rapid acceleration for a certain transient time an excessive amount of exhaust gas recirculation is provided for the diesel internal combustion engine. This can lead to the emission of an unacceptable amount of HC and CO and of other undesirable hydrocarbons such as soot and smoke in the exhaust gases of the diesel internal combustion engine.
It should be understood that this problem is not specifically a problem that occurs during high vehicle acceleration during conditions of high engine load. Rather, it is a problem which manifests itself during conditions of rapid increase of engine load, i.e. during rapid depression of the accelerator pedal of the vehicle, i.e. during conditions of high rate of change of acceleration, or high jerk, of the vehicle (jerk being defined as the differential coefficient of the engine load).